Page 233 - Atlas of Small Animal CT and MRI

Neoplasia 223

with peripheral or nonuniform contrast enhancement. hemorrhage. Ependymomas may be accommodated
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4,5
Tumor‐related production of vascular permeability by gradual ventricular dilation, hence edema is usually
factors, such as vascular endothelial growth factor, may absent or minimal, unless the tumor invades the perive­
also contribute to contrast enhancement in high‐grade ntricular brain parenchyma or hydrocephalus causes
gliomas. 6 periventricular interstitial edema.
The appearance of oligodendrogliomas on unen­ Ependymomas are typically isoattenuating on
hanced CT images is similar to that of astrocytomas. unenhanced CT images, although they can have a
They are typically hypoattenuating, and mass margins heterogeneous appearance.
may be ill defined or lacking, particularly when Ependymomas appear slightly T1 hypointense to
peritumoral edema is present. Marked central hypoat­ slightly hyperintense on unenhanced images and mod­
tenuation of some oligodendrogliomas, caused by the erately to markedly T2 hyperintense.
high water content of the mucinous core, may increase Contrast enhancement is usually marked and may be
the index of suspicion for this tumor. heterogeneous, which reflects the coarse texture of the
Oligodendrogliomas are moderately T1 hypointense tumor parenchyma, on both CT and MR images. 27,30,31
and markedly T2 hyperintense, specifically when there is Heterogeneity may be even more pronounced when
significant central mucinous content. Peritumoral edema cysts or hemorrhage are present. Tumor margins are
ranges from minimal to moderate, although even large typically distinct, because the majority of the mass
oligodendrogliomas may induce little edema formation. extends into a ventricular lumen (Figure 2.8.13).
Contrast enhancement of oligodendrogliomas on both
CT and MR images is highly variable, ranging from none Choroid plexus tumors
to marked, and when present it is often peripheral or non­ Choroid plexus tumors (CPT) are relatively common
uniform. Focal or regional contrast enhancement is often neoplasms that arise from the choroid plexus epithelium
distributed centrally or eccentrically within the greater within the lateral, third, and fourth ventricles and the lat­
tumor volume and may have a serpentine shape. 21,27–29 eral recesses. About 50% originate in the fourth ventricle
Although high‐grade oligodendrogliomas tend to con­ or lateral recesses. The average age of dogs at diagnosis is
trast enhance to a greater degree than low‐grade tumors, 6 years, which is earlier than most other intracranial
as with astrocytoma, this imaging feature is not a reliable tumors. Golden Retrievers appear to be highly overrep­
indicator of biological grade (Figure 2.8.11, 2.8.12). resented. Classification of canine CPT distinguishes
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choroid plexus papillomas (CPP), comparable to WHO
Mixed glial cell tumors grade I and morphologically benign, from choroid
Canine mixed glial tumors are usually comprised of plexus carcinomas (CPC), comparable to WHO grade
III and histologically more abnormal and more likely
tumor cells with both astrocytic and oligodendrocytic to invade the brain or give rise to intraventricular
features, or they may contain a combination of astrocytic or intrathecal metastases. Mild to moderate edema is
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and oligodendrocytic subpopulations. These tumors present in about 45% of CPP and about 70% of CPC. 32
have MR imaging features similar to those of astrocyto­ Choroid plexus tumors share many CT and MR
mas and oligodendrogliomas.
imaging features with ependymomas. Initially, they may
conform to the shape of the ventricle in which they grow,
Ependymal tumors but enlargement may lead to hydrocephalus because of
Ependymomas are uncommon tumors that arise from ventricular obstruction or, possibly, overproduction of
the ependymal lining cells of the ventricular system and CSF. Tumors have variable attenuation on unenhanced
thus may occur within the ventricular system of the CT images and may be T1 hypo‐, iso‐, or hyperintense
brain and spinal cord. Ependymomas usually affect and T2 hyperintense on MR images. Choroid plexus
older dogs and cats without breed predisposition. These tumors often appear heterogeneous, particularly when
tumors are predominantly intraventricular, although there is intratumoral hemorrhage.
some invade the adjacent brain parenchyma, and they Choroid plexus tumors usually show marked, uniform
expand to fill the ventricular cavity in which they arise, enhancement on both CT and MR images following
causing distortion of the ventricle and obstructive contrast administration, which reflects the underlying
hydrocephalus, depending on their size and location. papillary vascular architecture of these tumors
Ependymomas may be well differentiated (WHO grade (Figure 2.8.14). Intraventricular and intrathecal “drop
II) or anaplastic and aggressive (WHO grade III). metastases” may appear as intensely contrast‐enhancing
Grossly, the tumors can be soft, lobular (papillary type), foci in the ventricles or subarachnoid space
or solid (cellular subtype) and may contain cysts and/or (Figure 2.8.15). Choroid plexus papillomas and

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